A file/print server can be handy if you have a lot of other pc's that need to have a place to dump backups to, and on occasion burn a DVD of those backups for offline storage. Or, if you are going the mythtv route, it can also be a repository of dvd-backups or pix or whatever, why not both a mythtv backend and a samba windows share for backups.
Also consider a secure firewall / gateway, even if you have a hardware router it can be invaluable for remote access into your local home network, and want to have secure sessions and internet browsing from a remote location - Just set up any version linux, and set up squid-proxy listening on localhost and a secondary port for the ssh daemon other than the port 22 default (like 2200 or whatever) then set up a port-forward on your router for 2200-external to 2200-internal to whatever static ip-address on your internal net - and when you are at a public internet cafe, or at work and want to secure your internet browsing just create an ssh session and use the port-forward option like "ssh -p {alt ssh port} 13128:localhost:3128 user@homenetwork.com" (dyndns, noip, or other dyanmic internet ip-addressing is handy) and then set up your browser for a localhost proxy with 13128 as the port, and you have a poor-mans secure browsing facility whever you are at on the internet - all your browsing traffic is forwarded across the ssh tunnel to your home network, and not visible locally (at the internet cafe, or at work or at your in-laws, wherever) since ssh encryption is DES3 it will be difficult (not impossible) to decipher. OpenVPN is another robust port-forwarding type of client/server app that is much more flexible.